References published since 2 Jun. 2004 are mentioned to show current thinking concerning silver alloys and investment casting and in some cases to show uncontroversial matters of technical fact, but are not admitted as prior art.
Investment casting of sterling silver and standard deox alloys is reviewed by Jörg Fischer-Bühner, Silver casting revisited: the alloy perspective, The Santa-Fe Symposium 2010, the contents of which are incorporated herein by reference. However, to facilitate understanding of the historical development of significant silver alloys for investment casting and other purposes, patent specifications are discussed in the order of their earliest priority dates which are given after the name of the first listed inventor. It has not been convenient to preserve this chronological order for published literature in which the significance of the patented alloys is discussed.
It has long been desired to produce investment castings in silver with a bright and shiny as-cast colour. So-called “de-ox” sterling silver alloys are available inter alia from United Precious Metal Refining, Inc. (“UPM”) which claims on its website to have the only available silicon-deoxidized sterling silver casting grains and which are said to have the advantages of castability, reduced porosity, absence of firescale and tarnish resistance.
U.S. Pat. No. 4,973,446 (Bernhard I, UPM, 1990) explains that molten silver can absorb 22 times its volume of oxygen, so that molten silver when close to saturation has an oxygen content of about 0.3 wt %, and further explains that copper has a high affinity for oxygen forming cuprous or cupric oxide. Unless air is excluded during the casting process, standard Sterling silver castings may suffer from gas porosity and firestain. A problem with which the inventors were concerned was therefore to provide a silver alloy composition which exhibited reduced porosity when recast (e.g. from casting grain), which substantially reduced the formation of firescale in the casting process and which exhibited reduced grain size. As noted e.g. by Fischer-Bühner, Advances in the Prevention of Investment Casting Defects Assisted by Computer Simulation, Santa-Fe symposium, 2007 (the contents of which are incorporated herein by reference) the investment material has “tremendously low thermal conductivity” compared to all casting alloys independent of their chemical composition, which leads to solidification times of ˜90 seconds in the sphere part of a standard ring model for standard Sterling silver (FIG. 1), and consequently increased grain growth and reduced hardness compared to ingot-cast silver. The disclosed solution was an alloy consisting essentially of the elements set out in the table below. The alloy was said to produce castings free of normal firescale, with the additional advantages of greatly-reduced porosity and a reduced grain size leading to reduced labour in finishing and a reduced rejection rate of recast articles.
In the Bernhard I alloys, silver is present in the necessary minimal percentage to qualify as either coin silver or sterling silver, as appropriate. Copper (2.625 wt %) is added as a conventional hardening agent for silver as well as the main carrying agent for the other materials. Zinc is added to reduce the melting point of the alloy, to add whiteness, to act as a copper substitute, as a deoxidant, and to improve fluidity of the alloy. Tin is added to provide tarnish resistance, and for its hardening effect. Indium is added as a grain refining agent and to improve the wettability of the alloy. Silicon (0.1 wt %) acts as a deoxidant that reduces the porosity of the recast alloy and has a slight hardening effect. Boron is added to reduce the surface tension of the molten alloy and to allow it to blend homogeneously. A typical composition comprised 92.5 wt % silver, about 0.5 wt % copper, about 4.25 wt % zinc, about 0.48 wt % tin, about 0.02 wt % indium, about 1.25 wt % of a boron-copper alloy containing 2% boron and 98% copper, and 1% of a silicon-copper alloy containing about 10% silicon and about 90% copper. There is no disclosure or suggestion that silicon should be used as a deoxidant in the absence of zinc or at low levels of zinc.
U.S. Pat. No. 5,039,479 (Bernhard II, 1990) describes a master metal composition for making alloys of the above type, tin apparently being optional. An alloy used as a reference example in EP-B-0752014 (Eccles I) and said to be made in accordance with Bernhard II consists of silver 92.5 wt %, copper 3.29 wt %, zinc 3.75 wt %, indium 0.25 wt %, boron 0.01 wt % and silicon 0.2 wt %; it is reasonable to conclude that this is an analysis of a commercial alloy of UPM. Again there is no disclosure or suggestion that silicon should be used as a deoxidant in the absence of zinc or at low levels of zinc content.
As previously explained, the above mentioned disclosures concerning deox alloys should not be interpreted as disclosing the use of silicon as an individual element. Fischer-Bühner 2010 discloses in relation to zinc that together with silicon it serves as a deoxidant. As is apparent from the table below which is reproduced from Fischer-Bühner 2010, Si-containing deox alloys all contain large amounts of zinc. If UPM and other manufacturers had been able to obtain bright castings with less zinc or without zinc, they would have done so because zinc (b.p. 907° C.) is volatile at silver casting temperatures (˜1000° C.), reduces hardness and gives rise to gas porosity and shrinkage porosity.
CategoryAlloy codeSiliconZincCommentHigh Si-Arg-Deox+++++++Highest fluidity, firestaincontentand oxidation resistance andreduction of tarnish rateLow toSF928CHA++++++Medium-to-high firestainmediumAG113MA+++++and oxidation resistance,Si-contentAG114MA+++reliability and user-friendlinessSi-freeS925PHA− no −+++Most easy-to-cast andS925PTA− no −+forgiving, universal usage,high productivity
Patent GB-B-2255348 (Rateau, 1991) discloses a silver alloy that maintains the properties of hardness and lustre inherent in Ag—Cu alloys while reducing problems resulting from the tendency of the copper content to oxidise. The alloys are ternary Ag—Cu—Ge alloys containing at least 92.5 wt % Ag, 0.5-3 wt % Ge and the balance, apart from impurities, copper. The alloys are stainless in ambient air during conventional production, transformation and finishing operations, are easily deformable when cold, are easily brazed and are said not give rise to significant shrinkage on casting. They also exhibit superior ductility and tensile strength. Germanium exerts a protective function that is responsible for the advantageous combination of properties exhibited by the new alloys, and is in solid solution in both the silver and the copper phases. The microstructure of the alloy is said to be constituted by two phases, a solid solution of germanium and copper in silver surrounded by a filamentous solid solution of germanium and silver and copper which itself contains a few intermetallic Cu—Ge dispersoids. The germanium in the copper-rich phase inhibits surface oxidation of that phase by forming a thin GeO and/or GeO2 protective coating that prevents firestain during brazing and flame annealing. Furthermore the development of tarnish is appreciably delayed by the addition of germanium, the surface turning slightly yellow rather than black and tarnish products being easily removed by ordinary tap water. The alloy is useful inter alia in jewelery and silversmithing. Conventional grain-refining agents were tested, the specific materials evaluated or suggested being gold, nickel, manganese or platinum. Investment casting of the alloy was not reported.
As a result of discussions with Melvin Bernhard of UPM, Anthony Eccles of Apecs Investment Castings Pty Ltd developed alloys disclosed in EP-B-0752014 (Eccles I, 1993) for which the broadly claimed ranges of constituent elements is set out in the Table below. As explained in Anthony Eccles, The Evolution of an Alloy, The Santa-Fe Symposium, 1998 the alloy marketed by UPM was firescale-free on casting, but in its as-cast state it was too soft for most jewelery purposes and it did not harden appreciably. The present applicants consider that a hardness of 65-70 HV is needed for jewelery. The way these disadvantages were expressed in Eccles I was that the Bernhard I and Bernhard II alloys exhibited poor work hardening properties and did not achieve the mechanical strength of worked goods in traditional sterling silver. That disadvantage was disclosed as being overcome by addition of germanium to silver alloys of high zinc content broadly similar to those of Bernhard, the germanium-containing alloys reportedly having work hardening characteristics comparable to those of conventional 925 Sterling alloys together with firescale resistance. Zinc was said to influence the colour of the alloy and to act as a reducing agent (i.e. deoxidiser) for silver and copper oxides. Silicon was said to provide firescale resistance and to maintain good colour. Indium and boron could be provided for modification of rheology, reduction in surface tension and grain refinement. Exemplified alloys contained 2-3 wt % zinc and 0.15-0.2 wt % Si together with boron indium and germanium. The present inventors believe that Eccles was also driven to maintain high levels of zinc in the alloy by the need to avoid firestain at the time of casting, the problems created by high levels of zinc being such that if he had found any other way of achieving the same effects in a satisfactory alloy, he would have done so.
Eccles I was silent about the casting conditions employed. A skilled person is aware that as-cast hardness is dependent upon casting conditions. The present inventor has inferred that the figures quoted by Eccles are for ingot casting where cooling is very rapid and there is little opportunity for grain growth, cast ingots normally being rolled as in the experiments reported by Eccles and work hardening alluding to the manufacture of sheet and wrought products. As noted e.g. by Fischer-Bühner, Advances in the Prevention of Investment Casting Defects Assisted by Computer Simulation, Santa-Fe symposium, 2007 (the contents of which are incorporated herein by reference) the investment material has “tremendously low thermal conductivity” compared to all casting alloys independent of their chemical composition, which leads to solidification times of ˜90 seconds in the sphere part of a standard ring model for standard Sterling silver, and consequently increased grain growth and reduced hardness compared to ingot-cast silver. The hardness of APECS Bright Silver 925 said to be made in accordance with Eccles I (Ge content 0.2-0.3 wt %) is very significantly less than standard Sterling when investment cast with HV<50. The cast metal said to work harden to >160 HV at 75% rolling reduction, and is said to age harden to 120 HV by heating to an annealing temperature of 700° C. and quenching. It does not age harden without heating to an annealing temperature and quenching because of its low germanium content. Eccles I made no reference to investment casting. Insofar as APECS Bright Silver 925 is concerned a skilled person would regard the HV as investment cast as too low to be practical and would reject the age hardening route as involving conditions of a severity that are impractical for investment cast products owing to cracking and deformation, and for example would be impossible for products where stones are cast in place. Eccles I, therefore, does not solve the problem of providing an alloy that is practical for lost wax investment casting applications.
WO 96/22400 (Eccles II, 1995) refers to Eccles I and implicitly confirms the softness of the alloys of Eccles I insofar as it explains that for some alloys an increased copper content is required for increased hardness. It therefore aimed to provide high-copper alloys that exhibited reduced firescale, reduced porosity and oxide formation and reduced grain size relative to standard sterling silver. The disclosed solution was to provide alloys having the general composition set out in the table below, optional constituents being in brackets. It will be noted that the novelty over Eccles I was the absence of zinc, although high tin contents were considered acceptable. The specification explained that high copper alloys are inherently firescale-prone and that to create a high copper content, firescale-free sterling silver was unexpected. In particular it was unexpectedly found that the choice of deoxidizing additive (silicon) provided the facility of high copper content without significant firescale production, whereas the more common aggressive deoxidizers such as zinc did not. Firescale resistance was considered to be of particular importance for hot working to impart hardness and the use of germanium as an alloying agent provided alloys which were both firescale resistant and work hardenable and which were harder than prior art alloys due to their elevated copper content. Rheology-modifying additives such as indium and boron were optional ingredients but the ability of boron to act as a grain refiner had not yet been disclosed and its importance was not noted. Disclosed embodiments were Ag—Cu—Ge—Si and Ag—Cu—Ge—Si—In alloys and there was no boron-containing embodiment, a reference to fewer components providing the added advantage of a more stable grain structure teaching away from the addition of boron. The only exemplified alloys contained 0.2-0.3 wt % Si and 0.2-0.3 wt % Ge.
The Eccles II alloys were never developed into a commercial product despite their apparently desirable properties. One reason may be an insufficient level of germanium in the exemplified materials to give rise to the desirable properties in terms of firescale resistance, tarnish resistance and hardness associated with that element. There would have been a propensity for crack development especially when investment casting owing to the relatively high silicon content. The absence of boron would have hindered grain refinement so that investment castings in the Bernhard II alloy would have been unacceptably soft. None of Bernhard I, Bernhard II, Eccles I and Eccles II discloses or suggests a solution to these problems. Furthermore, Eccles II is completely silent about lost wax investment casting about and the repeated mention of platework, rolling and work hardening teaches away from the use of these alloys for lost wax investment casting.
U.S. Pat. No. 6,168,071 (Johns, 1998) describes and claims inter alia a silver/germanium alloy having an Ag content of at least 77% by weight, a Ge content of between 0.5 and 3% by weight, the remainder being copper apart from any impurities, which alloy contains boron as a grain refiner at a concentration of up to about 20 ppm. The boron is provided as a copper-boron alloy e.g. containing 2 wt % boron and imparts greater strength and ductility to the alloy and permitting strong and aesthetically pleasing joints to be obtained using resistance or laser welding. It was explained that grain refining silver alloys had proved difficult and that a person of ordinary skill in the art would not previously have considered boron for this purpose, and that it is effective in inhibiting grain growth even at soldering temperatures. Again investment casting of the alloy was not reported.
EP-B-1631692 (Johns II) discloses firestain and tarnish-resistant ternary alloy of silver, copper and germanium containing from more than 93.5 wt % to 95.5 wt % Ag, from 0.5 to 3 wt % Ge and the remainder, apart from incidental ingredients (if any), impurities and grain refiner, copper. Investment casting of strip is reported and the strip is said to be free of hot short (cracking) defects. The appearance of the strip as cast was not evaluated. Although the bracketed ingredients in the table below were optionally present as a hypothetical possibility, in practice alloys containing them were not made or tested.
Eccles 1Eccles IIRateauJohnsJohns IIElementBernhard I wt %wt %wt %wt %wt %wt %Ag  89-93.5>90To 100%≧92.5≧92.593.5-95.5Cu0.5-6  0.5-6   2.5-19.54.5-7.24.5-7.2balanceGeN/A0.01-1  0.01-3.3 0.5-3  0.5-3  0.5-3  Zn0.5-5  2-4(0.5)Tin0.25-2  0-6(0-6)(0.5)In0.01-1.25  0-1.5  (0-1.5)(trace)Si0.01-2  0.02-2  0.02-2  (0.1-1)  B0.01-2  0-2(0-2)≦20 ppm1-40 ppm
Various alloying ingredients are discussed by Fischer-Bühner in his 2010 paper which reflects current practice in the casting of alloys other than those which contain germanium.
Copper remains the main addition in variations of standard sterling silver despite its many disadvantages. It accelerates tarnishing. It lowers the melting point of silver and leads to a broad melting range, making the alloy intrinsically prone to hot cracking. It oxidizes easily, leading to dark surface oxide layers on as-cast trees during cooling in air after pouring or during re-heating, e.g. for soldering. It also leads to internal or subsurface oxidation which can be revealed as “firestain” (grey, bluish or reddish areas) on finished surfaces.
Zinc is used up to ˜2.5 wt %. It decreases the surface tension of the melt, increases fluidity and form filling and reduces surface roughness. Together with silicon it helps to avoid the development of dark copper oxide layers and firestain. However, the high vapour pressure of zinc can lead to loss of Zn by evaporation depending on melting conditions and to fumes of zinc.
Silicon is used up to ˜0.2 wt %. It has a greater affinity for oxygen than silver, copper and zinc and therefore acts as deoxidizer of the molten alloy, but depending on equipment and process conditions it can also give rise to surface dross. It prevents the formation of dark copper oxide layers by preferential formation of bright and white silicon-oxide layers on as-cast trees. Like zinc it increases fluidity and assists in form filling. It also widens the melting range and tends to segregate and form low-melting phases along grain boundaries, leading to increased risk of hot cracking. If used in high quantities, silicon and zinc may reduce the rate of tarnishing.
A bright and shiny as-cast tree colour is often a practical necessity, especially for companies carrying out stone-in-place casting. In such cases alloys with medium to high silicon level are at present considered by Fischer-Bühner the only safe choice (this statement being made in relation to alloys containing zinc and silicon but not germanium). While the dark copper oxide layers on as-cast tree surfaces obtained for silicon-free alloys can be removed by pickling, they are sometimes difficult to remove completely below the stones. A high silicon-level provides the most bright as-cast tree colour under all manufacturing conditions and the most white metal colour after finishing, making it particularly attractive for stone-in-place casting. Furthermore the higher fluidity of such an alloy allows for lower flask temperatures, which reduces the risk of damage to the stones
Depending on alloy composition the brightness of as-cast trees also significantly depends on the cooling procedure of flasks after pouring. A common standard cooling procedure consists in removing the flask from the flask chamber ˜1 min after pouring followed by cooling in air for another 10-20 min before quenching. For silicon-free alloys the surface of the as-cast tree then is covered by a grey to dark copper-oxide layer depending on flask temperature. The oxidation can be drastically reduced if a flask is kept for an extended time (e.g. 3-5 min) in the flask chamber under vacuum or protective gas which then is followed by removal of the flask from the machine and immediate quenching. In this case just a slight grey, sometimes yellowish discoloration is observed and internal (subsurface) oxidation of the copper in the alloy is avoided which eliminates firestain for Si-free alloys and significantly improves scrap metal quality. For Si-containing alloys such a process modification is not significant, since the brightness of the as-cast tree is not much affected by different flask cooling procedures. However, more protected cooling reduces consumption of silicon and also improves scrap metal quality.
Especially for alloys with a broad melting range, like all 925 silver alloys, “hot cracking” or “hot tearing” can be a problem. Hot cracking mainly occurs when mechanical stress is acting on the metal during the final stages of solidification, hence when there is only a small amount of liquid metal left between the growing grains. The thermal shrinkage of the solidifying metal coupled with the thermal expansion of the investment material (heating up when in contact with the hot metal) exerts local stresses and tears the metal apart. Fischer-Bühner explains that silicon-containing alloys are more prone to hot-cracking than silicon-free alloys. The increased risk for hot cracking of silicon-containing alloys as compared to silicon-free alloys can be theoretically understood. Silicon tends to segregate to grain boundary areas during solidification where it eventually forms low melting phases. This broadens the melting range, from a width of typically ˜120° C. for silicon-free alloys to ˜150-170° C. for medium-to-high silicon levels and also increases solidification time. For example an item that would need 1.5 min for completion of solidification if cast in a silicon-free alloy at a flask temperature of 500° C. needs around 2.5 min if cast in an alloy with medium-to-high silicon-content. Hence the danger zone (temperature and time range) during which hot cracking may occur is broadened for silicon-containing alloys. A further problem with silver castings is shrinkage porosity to which silicon-containing alloys are more prone.